The present invention relates to an optical recording medium, specifically to a phase modulation type optical recording medium having excellent reproduction output characteristics, in which reproduction of recorded portions is performed by irradiating light in the range of 380 nm˜450 nm in wavelength λ and organic pigment material is used as a recording film.
Functional organic pigment material is heretofore widely used as recording material for write once type optical discs and are manufactured in large quantity at a low cost, particularly as write once type compact discs (CD-R).
Also, in optical systems in DVD (Digital Versatile Disc), a write once type has been standardized as a write once type DVD-R and has been on sale.
Moreover, as an optical disc of currently so-called a next-generation optical recording medium in which higher recording capacity, i.e., higher recording density is intended, there has been considered standardization of irradiating blue-violet light of shorter wavelength onto a recording surface through an object lens of 0.85 in numerical aperture N.A. from the side of a light transmission protective film formed on the recording surface.
In an optical recording medium in which the above-mentioned higher N.A. and shorter wavelength of reproduction light are obtained and a reproduction light is irradiated onto the recording surface from the side of a light transmissive protection film (hereinafter this optical recording medium is referred to as DVR for convenience), there are also increasing needs for a write once type optical recording medium (DVR) in which so-called archive recording is performed, i.e., recording is performed only once and the record is stably held over many years without being erased.
Although in the above-described optical recording medium (DVR) under consideration of such standardization the recording film based on phase change material is used, as the recording film it is preferable to use organic pigment material as in CD-R considering a simple method of manufacturing and cost reduction when the optical recording medium has a write once type construction.
However, there is a problem when knowledge of the conventional write once type CD-R and DVD-R is applied to the write once type optical recording medium (DVR) employing the above-mentioned short wavelength and high N.A.
Specifically, in an optical recording medium using a short wavelength of 380 nm˜450 nm, e.g., blue-violet light source of 405 nm±5 nm for high recording density, i.e., high resolution, if the recording film is comprised of organic pigment material, the characteristics of such organic pigment material constituting the recording film differ from those in CD-R and DVD-R and optical characteristics and optical conditions may be different, so that there may be problems in applying knowledge used in general CD-R and DVD-R.
For example, it is impossible to obtain excellent reproduction characteristics by such measures as modifying depth of recessed portions, for example, tracking grooves formed in the substrate of an optical recording medium in response to differences in wavelengths of irradiated lights.
Further, the track pitch in the above mentioned optical recording medium (DVR) is significantly smaller as compared to that in conventional optical recording media. The track pitch is, for example, 0.34 μm as compared to 1.6 μm in CD, for example. In this case, the slope of side walls in tracking groves formed in the disc substrate will be the matter.
In general, fine concavity and convexity such as grooves or pits in CDs and the like are formed while manufacturing the disc substrate by injection molding in which a stamper having a fine pattern of concavity and convexity corresponding to the grooves is used, by a 2P method (photopolymerization method) or by other methods.
The stamper is manufactured by forming the master, that is, mastering using photoresist, and blue light or ultra-violet light is generally used for pattern exposure onto such photoresist in the mastering. However, when such fine track pitches as those provided in the above mentioned DVR are narrow and small, the pattern exposure by the above mentioned light results in gentle slope of side walls of the grooves.
Accordingly, although there have been attempted such methods as performing pattern exposure onto the photoresist by electron beam lithography, by reducing a spot size with disposing a focusing lens adjacent to the exposure surface in an optical system, i.e., taking so-called near field construction or the like in order to sharpen the side walls of the grooves or to improve those conditions, studies on optimization of the side walls of the grooves are insufficient, and sufficient reproduced output has not yet been achieved.